Field of the Invention
The invention relates to a stacked heat exchanger, in particular welded, ferritic heat exchanger for high temperature applications.
Description of the Background Art
A stacked heat exchanger is disclosed, for example, by the applicant in DE 10328274 A1. Here, suitably contoured metal sheets, alternating with solder foils if applicable, are stacked—framed by cover plates—in a fixture, suitably pre-pressed, and welded to boxes while the tension is maintained. These boxes have the dual function of maintaining the preloading as a lost soldering device and ensuring the delivery of material to the heat exchanger. If no solder foils are going to be used, solder can also be provided in a variety of ways, including externally, namely before the boxes are welded on. The thus pretreated stacked heat exchanger is then sealed by soldering.
In addition, a stacked heat exchanger is disclosed by the applicant in DE 10 2007 056 182 A1 in which the internal heat exchanger block is mechanically separated by a decoupling device from the housing, which is sealed with respect to the outside. The decoupling device can be, for example, a mineral fiber mat or a molded knit wire mesh, with filling or film covering if applicable. It is disadvantageous here that although thermomechanical decoupling is ensured, leakage occurs from one flow to the other flow via the decoupling device, impairing heat transfer performance.
DE 10 2009 022 984 A1 discloses a heat exchanger that has a housing, made, e.g., of a Ni alloy, that is high temperature resistant relative to a soft core of, for example, ferritic stainless steels containing Al. The core is highly ductile in order to accommodate stresses during heating. The base material contains enough aluminum to minimize corrosion phenomena such as oxidation or Cr evaporation. The high strength and hot strength of the box material ensure that the component remains sealed to the outside, so that hydrogen cannot escape under any circumstances.
It is a disadvantage of the stacked heat exchangers known from the prior art, especially DE 10 2009 022 984 A1, however, that in an application in conjunction with an APU (Auxiliary Power Unit) and the long operating times there of 15 to 20 thousand hours, enough aluminum diffuses out of the aluminum-containing ferritic base material into the cover plate and box material made of Ni alloys to drop below the critical content of aluminum in the aluminum-containing ferrites needed to be able to produce the protective Al2O3 layer. This results in what is called breakaway (catastrophic) oxidation with the development of leaks in the stacked heat exchanger.